20 June 2008


Here is yet another Astronomical Term - Occultation. Sounds weird but a simple term. An occultation is an event that occurs when one object is hidden by another object that passes between it and the observer. Does it sound familiar? Yes! Eclipses and Transits are also known as Occultations.

Every object in space move in a particular direction. By coincidence any two objects may come in the same line of sight from Earth which causes Occultation. The most famous occultation is the Eclipse.

Total Solar Eclipse

In case of Solar Eclipse, the Moon comes in between the Sun and the Earth. Hence the Moon and the Sun are in the same line of sight. The Moon appears to block the Sun from the view or the Moon occults the Sun.

But this is not the case in the Lunar Eclipse. In Lunar Eclipse it is the Earth that occults the Moon when observed from the Sun and not from the Earth. Hence Lunar Eclipse cannot be called the Occultation.

Position of the Earth, Sun and the Moon during the Total Solar Eclipse (left) and the Lunar Eclipse (right)

The term Transit is used when one object appears to move across the face of another object. Usually one of the object is small when compared to the other. The most famous transits are that of Mercury and Venus over the Sun.

Transit of Mercury over the Sun

Transit of Venus over the Sun

Enlarged Image of the transit of Venus

Transit of Io (Satellite of Jupiter) over Jupiter

The other types of occultation are when a star occults another star or when a satellite occults another satellite. This can only be observed through telescopes.

Dione (Satellite of Saturn) occulting Rhea (satellite of Saturn)

Another rare type of occultation is when a planet occults another planet. It is not seen very often. Here is the list of occultation by planets that occured in the past and the ones which will occur in the future.

* 19 Sep 1702 - Jupiter occults Neptune
* 20 Jul 1705 - Mercury transits Jupiter
* 14 Jul 1708 - Mercury occults Uranus
* 4 Oct 1708 - Mercury transits Jupiter
* 28 May 1737 - Venus occults Mercury
* 29 Aug 1771 - Venus transits Saturn
* 21 Jul 1793 - Mercury occults Uranus
* 9 Dec 1808 - Mercury transits Saturn
* 3 Jan 1818 - Venus transits Jupiter
* 22 Nov 2065 - Venus transits Jupiter
* 15 Jul 2067 - Mercury occults Neptune
* 11 Aug 2079 - Mercury occults Mars
* 27 Oct 2088 - Mercury transits Jupiter
* 7 Apr 2094 - Mercury transits Jupiter
* 21 Aug 2104 - Venus occults Neptune
* 14 Sep 2123 - Venus transits Jupiter
* 29 Jul 2126 - Mercury occults Mars
* 3 Dec 2133 - Venus occults Mercury

A simulation of Venus transiting Jupiter, as it did on January 3, 1818.

Occultation can also occur in other types such as Moon occulting a star, Moon occulting a planet, Planet occulting a satellite, etc.

Minutes before the Moon occulted the star Albedaran

Before Moon occults Jupiter

Jupiter occulting Europa (Satellite of Jupiter)

The most recent occultation took place on 10th May, 2008 when the Moon occulted the planet Mars at around 8pm IST. It was clearly visible even at Chennai.